I think it obviously should be minus 13 degrees C.

although the transition temperature (below which grey tin is the stable allotrope) is 13.2 degrees C, in practice the transformation does not occur at an appreciable rate above about -15 degrees, reaching a maximum at about -30 degrees and becoming very small below -50 degrees. The transformation is catalysed by the presence of grey tin nuclei, but is strongly inhibited by the presence of 0.1% of lead, bismuth or antimony.
http://palimpsest.stanford.edu/byform/mailing-lists/cdl/1998/1204.html

The transformation is apparently called "tin plague" or "tin pest", by the way, and there appraently is still another tin allotrope ("rhombic tin") at high temperature.

I wonder what kind of a medal the guy who persuaded the Czar to have tin used got.
Some google sources indicate that this "buttons" story happened in World War II -- which I rather doubt.

PS to dr. bill: why "Dear Ken:"??